Electrodeless light sources which operate by coupling high frequency power to a high pressure arc discharge in an electrodeless lamp have been developed. These light sources typically include a high frequency power source connected to a termination fixture with an inner conductor and an outer conductor surrounding the inner conductor. The electrodeless lamp is positioned at the end of the inner conductor and acts as a termination load for the fixture. The termination fixture has the function of matching the impedance of the electrodeless lamp during high pressure discharge to the output impedance of the high frequency power source. Thus, when the high pressure discharge reaches steady state, a high percentage of input high frequency power is absorbed by the discharge in the electrodeless lamp.
Previous patents describe electrodeless light sources wherein the termination fixture couples power to one end of the electrodeless lamp. While light sources with single-ended coupling give generally satisfactory results, they have certain disadvantages. In the situation where power is coupled to one end of the lamp and the other end is open-circuited, the electric field in the lamp decreases with increasing distance from the power coupling conductor. As a result, arc intensity also decreases with increasing distance from the power coupling conductor. This gives rise to a non-uniform luminance.
Non-uniform arcs are undesirable for several reasons. They produce both hotspots and coldspots in the wall of the envelope. Hotspots occur adjacent to points of maximum arc intensity and at points where the arc attaches to the lamp envelope. The envelope wall material has a maximum operating temperature. Therefore, the total power which can be delivered to the lamp without exceeding the maximum temperature is reduced by the existence of hotspots. The light output of the lamp is correspondingly lowered. Moreover, for a given value of input power, the life of the lamp is reduced when hotspots occur. Coldspots occur at the points on the lamp wall which are most distant from the arc and are undesirable because fill material can condense on the lamp envelope at coldspots and can block a portion of the light output by absorption. Conversely, a more uniform arc results in a more uniform wall temperature and a higher level of input power and light output can be achieved. Also, the life of the lamp is increased when temperature variations over the wall of the lamp is minimized. Therefore, in powering electrodeless HID lamps, it is advantageous to deliver power at both ends of the tubular capsule to permit even heating of the lamp envelope.
U.S. Pat. No. 4,266,162, which issued to McNeill et al on May 5, 1981, describes an electromagnetic discharge apparatus having a coupling fixture which couples power to both ends of an electrodeless discharge vessel. Power is coupled to the fixture from either two high frequency power sources or from a single high frequency power source by using a power divider. Since phasing and power dividing is performed in the high power sections of the apparatus, increased costs and power handling requirements of the electronics are required.
U.S. Pat. No. 5,070,277, which issued to Lapatovich on Dec. 3, 1991, describes a dual-ended excitation scheme to deliver microwave power to a cylindrical lamp capsule used in an electrodeless lamp. A single microwave power source delivers power at levels of about 25 W to an applicator where it is divided and applied to both lamp ends via a microstrip balun.
Although the above-described methods for delivering power to both ends of a lamp have been employed with varying degrees of success, it has been discovered that certain disadvantages still exist. For example, difficulties associated with power imbalance exist which produce overheating of one end of the lamp. Moreover, non-uniform temperature distribution along the lamp envelope is produced which leads to condensate redistribution in undesired fashion. This leads to reduced lamp light output and accelerated attack by chemical fill species. For these reasons, an improvement in the power distribution and subsequent heating of the lamp envelope would be significant.